The present invention relates to a driving device and a driving method of an on-demand ink jet printer head, and in particular, to a driving device and a driving method of an on-demand ink jet printer head by which discharge of minute ink drops from orifices of the printer head can be executed correctly and stably according to image data, and thereby high quality printed output can be obtained with high stability.
Ink jet printers are generally classified into two types according to their ink drop discharging method: continuous ink jet printers and on-demand ink jet printers. In a continuous ink jet printer, the printer head constantly discharges ink drops continuously and only ink drops necessary for generating images are allowed to reach an object such as paper. In an on-demand ink jet printer, the printer head selectively discharges ink drops which are necessary for generating images.
The on-demand ink jet printer head is generally provided with orifices (nozzles) for discharging ink drops and a mechanism for controlling the discharging operation of the printer head from the orifices according to image data. In the on-demand ink jet printer head, a pressure generation means is generally provided to each ink chamber corresponding to an orifice, and ink drops are discharged from each orifice onto the object such as paper due to acoustic waves which are generated by the pressure generation means corresponding to the orifice. In the ink drop discharging method using the pressure generation means, at least a part of walls of the ink chamber is deformed by an electric-mechanic transducer (actuator) so as to generate acoustic waves, and ink drops are discharged by the acoustic energy of the acoustic waves.
In another ink drop discharging method, an electric-thermal transducer is provided to each ink chamber corresponding to an orifice. The ink is heated and vaporized into bubbles by the electric-thermal transducer so as to generate acoustic waves, and ink drops are discharged by acoustic energy of the acoustic waves.
In printers employing such ink drop discharging methods, a printer head unit having a plurality of orifices for discharging ink drops is mounted on a carrier with an ink cartridge. The carrier which is driven by a motor moves above the object such as paper in scanning movement. Along with the scanning movement of the carrier, ink drops are discharged from the orifices according to timing based on image data, and thereby an image according to the image data is generated on the object by the applied ink dots.
Some measures have been taken to realize high speed printing. The number of orifices has been increased so as to increase the number of simultaneously printed ink dots, and the ink discharge frequency (printing frequency) of each orifice (i.e. the driving frequency of the printer head) has been raised by optimizing the total configuration of the ink channel from the ink cartridge to the edge of the orifice.
The ink used for the ink jet printer is roughly divided into water-base ink and non-water-base ink. The water-base ink has the following advantages:
(1) The tolerances of physical properties (surface tension, viscosity, etc.) of the ink are wide and thus the stability of ink drop discharge properties can be made high.
(2) The main solvent is water and thus the change of physical properties due to volatilization does not occur much.
(3) Safe, stable, and easily handled.
(4) Good color development on paper.
On the other hand, the non-water-base ink has the following advantages:
(1) The materials for the printed object can be selected widely (iron, ceramic, etc.).
(2) Superior fixation on the printed object, little blur of ink.
(3) High water-resistance of prints.
The water-base ink is mainly used today for general and personal use, from the viewpoint of print quality, reliability of the printer head, etc.
In the water-base ink, coloring agents (dyes, pigments, etc.) and a variety of agents such as ink property control agents (surface-active agents, pH control agents, high-boiling-point agents) are mixed together. The agents are mixed together in a proper mixture ratio, in consideration of synergistic effects of the agents, physical properties of the ink, structure design of the printer head, etc.
The solubility in the main solvent varies depending on dyes, that is, depending on the color of ink, and thus agents such as die-solubility accelerator becomes necessary for some colors. Therefore, the type and the amount of additives vary depending on ink colors and thus the properties and stability of ink are necessitated to vary depending on colors. In order to raise the color density on the object and obtain vivid print, the amount of the coloring agent in ink has been increased.
The ink in the orifice of the ink jet printer head has a meniscus, that is, the interface of the ink to air. The meniscus has a concave curve which is caused by surface tension of the ink and a negative pressure of the ink cartridge. Evaporation of volatile ingredients of the ink easily occurs around the meniscus and thereby the properties of the ink tend to change around the meniscus. Especially, the viscosity of the ink tends to increase near the meniscus.
Due to the change of ink properties, the statuses of discharged ink drops are easily deteriorated when the ink is discharged after a long pause. Especially, change in the size of an ink drop, decrease of ink drop speed, variations in flying direction of ink drops, etc. occur, and thereby the quality of print is necessitated to be deteriorated. If the statuses of the ink around the meniscus gets worse, the orifice is stopped up with ink and ink discharge becomes difficult or impossible, causing fatal flaws in the printed output.
Some printer head maintenance methods have been proposed for avoiding the change of ink properties in the orifices (especially, the ink coagulation due to evaporation of volatile ingredients). In the following, a few conventional techniques for avoiding the ink coagulation due to change of ink properties will be explained.
In a conventional xe2x80x9cink jet printing methodxe2x80x9d disclosed in Japanese Patent Application Laid-Open No.SHO57-61576, each piezoelectric element which is used for discharging ink drops is driven also in pause periods with a voltage that is lower than a threshold voltage necessary for discharging the ink drops. In the conventional technique, an electric-mechanic transducer (i.e. the piezoelectric element) which is used for discharging ink drops is driven by the lower voltage (by which ink drops are not discharged) so as to give subtle vibration to the meniscus, and thereby the ink coagulation is prevented.
A conventional ink jet printing device is provided with an energy increase means which increases initial ink discharge energy in the case where ink drop discharge is restarted after a discharge pause period that is longer than a predetermined period. In the conventional technique, the ink discharge energy when the ink drop discharge is restarted is increased so that the properties of discharged ink drops such as speed, size, etc. will be close to those in continuous ink drop discharge operation.
The above conventional techniques could be employed for discharging ink drops stably in the case where the size of an ink drop is larger than the size of the orifice, since the ink drop discharge properties could have enough margins, that is, since a little environmental change did not exert effects on the ink drop discharge properties. However, the conventional techniques can not be employed for discharging ink drops stably and uniformly in the case where ink drops smaller than the diameter of the orifice are discharged by means of meniscus position control and acoustic wave control. In such cases, if the pause continues longer than a certain period, the ink drop discharge properties deteriorate considerably (decrease in speed and size of the ink drop, unstable flying statuses of the ink drops, etc.) causing shift of ink dot positions, uneven diameter of ink dots, dispersed ink dots due to unstable satellite ink drops, etc., and thereby the quality of printed image is deteriorated. In the meniscus position control and the acoustic wave control which are employed for discharging minute ink drops, precise control of ink properties is required for executing the meniscus position control and the acoustic wave control properly and thereby discharge minute ink drops correctly.
Even in the case of the water-base ink whose main solvent is water, the water-base ink used for ink jet printers shows slight thixotropic behavior. When a pause of the ink drop discharge (even a short pause in the scanning movement of the printer head between ink refresh operations) existed, ink flow in the ink channel changes and the thixotropic behavior of the ink in the meniscus position control emerges in a level that can not be neglected. The conventional techniques have not considered such thixotropic ink properties and thus could not realize stable discharge of minute ink drops. As the cause of the change of the ink flow in the ink channel, two states of the ink can be considered: a first state in the continuous ink drop discharge operation in which ink is constantly supplied from the ink pool to the orifice through the ink supply hole and the ink chamber, and a second state in the pause periods in which the ink drop discharge and the ink supply is stopped and thereby the ink flow becomes 0.
Further, in a plurality of to-and-fro action of the printer head, the viscosity of the ink around the meniscus raises locally due to evaporation of the volatile ingredients, thereby the energy applied for the meniscus position control and the acoustic wave control is dissipated, and thereby variations occur in the behavior of the meniscus and the formation of the ink column (an initial state of the ink drop including ink tail before being separated from the meniscus). In the continuous ink drop discharge operation in which ink discharge and ink refill are executed continuously, ink around the meniscuses is constantly replaced by ink supplied from the ink pool through the ink supply hole and the ink chamber. On the other hand, the ink replacement is not executed in the pause periods, thereby the local increase of the ink viscosity around the meniscus is caused.
Furthermore, when the ink jet printer is required to execute gradation printing, more precise ink drop control for generating minuter and uniform ink drops is required of the ink jet printer head. Therefore, it becomes more difficult to realize stable ink drop discharge for gradation printing by the conventional techniques.
The first conventional technique can hardly be applied to the case where minute ink drops are discharged by means of the meniscus position control and the acoustic wave control, since the vibration, which is given to the meniscuses or the ink chambers by the voltage lower than the threshold voltage necessary for ink drop discharging, is not enough for avoiding the deterioration of the ink drop property after pause periods. Little effect can be obtained even if the voltage is changed below the threshold voltage with a fast changing rate so as to give rapid change of the volume and pressure to the ink chamber, and thus ink drop discharge properties of desired precision can not be attained. In the case where the minute ink drop smaller than the diameter of the orifice is discharged by means of the meniscus position control and the acoustic wave control, the displacement of the piezoelectric element necessary for discharging ink drops is extremely small, therefore, only little displacement of ink can be attained by the voltage lower than the threshold voltage necessary for discharging ink drops, and thus the ink properties can not be maintained properly to be able to realize ink drop discharge properties equivalent to those in the continuous ink drop discharge operation. Therefore, stable minute ink drop discharge after pauses can not be attained by the subtle vibration. According to studies by the present inventors, large and a little slow displacement of ink is more effective than the rapid and subtle vibration, for the restart of the minute ink drop discharge.
Further, by the subtle vibration which is continuously given to the ink chamber and the meniscus in the pause periods with the driving frequency of the printer head, the number of (additional) actions of the piezoelectric element is increased much, and thereby the operating life of the printer head including the piezoelectric elements is necessitated to be short. When voltage is applied to the piezoelectric element, strain occurs in a piezoelectric material which is filled between electrodes of the layered piezoelectric element. In the case of the layered piezoelectric element, inactive parts, in which the internal electrodes are not paired (positive electrodes only or negative electrodes only), necessarily exist in the piezoelectric element on account of wiring. The inactive part does not exhibit piezoelectric effect, and thus large internal stress occurs at the interface between the active part and the inactive part. The continuous subtle vibration causing internal stress shortens the operating life of the piezoelectric element. Further, there is a possibility that the vibration of the piezoelectric element causes damage (splitting or peeling of the plate layers, etc.) to the layered head, shortening the operating life of the printer head. Also in the case of a bimorph element, stress caused by bending of the inactive plate and the piezoelectric plate decreases the life of the bimorph element.
A conventional xe2x80x9cink jet printing devicexe2x80x9d disclosed in Japanese Patent Application Laid-Open No.HEI9-30007 starts applying low voltage pulses (that are lower than a threshold voltage necessary for the ink drop discharge) to the piezoelectric element when a driving cycle of the printer head passed after the last ink drop discharge, so as to give the minute vibration to the ink meniscus, and stops the application of the low voltage pulses when a predetermined period passed. In the conventional technique, the application of the minute meniscus vibration is stopped after a predetermined period in order to avoid coagulation of ink due to evaporation of ink solvent. The continuous meniscus vibration promotes evaporation of ink solvent, causing coagulation of ink and stop-up of the orifices. The conventional technique avoids the stop-up of the orifices with ink by limiting the meniscus vibration period. Due to the limitation of the meniscus vibration period, the aforementioned problem with respect to the operating life of the ink jet printer head is also avoided. However, after the stop of the meniscus vibration, the ink drop properties keep on being deteriorated and no countermeasures are taken. Further, the minute meniscus vibration is not so effective for maintaining the ink properties in the discharge pause periods as a large and a little slow ink displacement, as mentioned above.
It is therefore the primary object of the present invention to provide a driving device and a driving method of an on-demand ink jet printer head which discharges minute ink drops by means of the meniscus position control and the acoustic wave control, by which uniform and stable ink drop discharge properties can be realized when ink drop discharge is restarted after a discharge pause period during scanning movement of the ink jet printer head, and thereby high quality printed output can be obtained with high stability.
Another object of the present invention is to provide a driving device and a driving method of an on-demand ink jet printer head, by which uniform and stable ink drop discharge can be realized without shortening the operating life of the electric-mechanic transducer for generating the acoustic wave and the operating life of the head layered body, along with reducing power consumption of the ink jet printer.
In accordance with a first aspect of the present invention, there is provided a driving device of an on-demand ink jet printer head for driving a printer head of an on-demand ink jet printer according to image data and thereby letting the ink jet printer head discharge ink drops from its orifices so that printing according to the image data will be executed. The driving device comprises a discharge voltage waveform application means, a discharge pause period length determination means, a pre-discharge voltage waveform determination means and a pre-discharge voltage waveform application means. The discharge voltage waveform application means generates discharge voltage waveforms corresponding to each orifice of the ink jet printer head based on the image data, and applies the discharge voltage waveforms to electric-mechanical transducers corresponding to each orifice so that ink drop discharge will be executed according to the image data due to volume change of each ink chamber corresponding to each orifice caused by movement of the electric-mechanical transducer and thereby an image according to the image data will be printed. The discharge pause period length determination means determines the length of each discharge pause period with respect to each orifice based on the image data. The pre-discharge voltage waveform determination means determines or selects a pre-discharge voltage waveform to be applied to the electric-mechanical transducer before restart of ink drop discharge after the discharge pause period, based on the length of the discharge pause period which has been determined by the discharge pause period length determination means. The pre-discharge voltage waveform application means applies the pre-discharge voltage waveform which has been determined by the pre-discharge voltage waveform determination means to the electric-mechanical transducer.
In accordance with a second aspect of the present invention, in the first aspect, the pre-discharge voltage waveform determination means sets voltage difference in the pre-discharge voltage waveform larger than voltage difference in the discharge voltage waveform.
In accordance with a third aspect of the present invention, in the first aspect, the pre-discharge voltage waveform determination means calculates a discharge pause cycle number by dividing the length of the discharge pause period by the driving cycle of the ink jet printer head, and determines or selects the pre-discharge voltage waveform based on the calculated discharge pause cycle number.
In accordance with a fourth aspect of the present invention, in the first aspect, the pre-discharge voltage waveform is applied to the electric-mechanical transducer just before the restart of the application of the discharge voltage waveform after the discharge pause period.
In accordance with a fifth aspect of the present invention, in the fourth aspect, the pre-discharge voltage waveform is applied to the electric-mechanical transducer a printer head driving cycle before the restart of the application of the discharge voltage waveform.
In accordance with a sixth aspect of the present invention, in the fourth aspect, the pre-discharge voltage waveform is applied to the electric-mechanical transducer two printer head driving cycles before the restart of the application of the discharge voltage waveform.
In accordance with a seventh aspect of the present invention, in the first aspect, the pre-discharge voltage waveform determination means executes the determination or selection of the pre-discharge voltage waveform so that ink meniscus vibration due to the application of the pre-discharge voltage waveform will be attenuated enough before the restart of the application of the discharge voltage waveform.
In accordance with an eighth aspect of the present invention, in the first aspect, the voltage of the pre-discharge voltage waveform is gradually increased from the bias voltage of the electric-mechanical transducer during the discharge pause period so as to decrease the volume of the ink chamber gradually and thereafter returned to the bias voltage just before the restart of the application of the discharge voltage waveform.
In accordance with a ninth aspect of the present invention, in the first aspect, the voltage of the pre-discharge voltage waveform is gradually decreased from the bias voltage of the electric-mechanical transducer during the discharge pause period so as to increase the volume of the ink chamber gradually and thereafter returned to the bias voltage just before the restart of the application of the discharge voltage waveform.
In accordance with a tenth aspect of the present invention, in the ninth aspect, the pre-discharge voltage waveform in the return to the bias voltage is set so that ink drop discharge will not be caused by decrease of the volume of the ink chamber.
In accordance with an eleventh aspect of the present invention, in the first aspect, the voltage of the pre-discharge voltage waveform is decreased below the bias voltage of the electric-mechanical transducer so as to increase the volume of the ink chamber and thereafter returned to the bias voltage just before the restart of the application of the discharge voltage waveform.
In accordance with a twelfth aspect of the present invention, in the eleventh aspect, voltage difference in the pre-discharge voltage waveform is set based on the length of the discharge pause period.
In accordance with a thirteenth aspect of the present invention, in the first aspect, the voltage of the pre-discharge voltage waveform is increased above the bias voltage of the electric-mechanical transducer so as to decrease the volume of the ink chamber and thereafter returned to the bias voltage just before the restart of the application of the discharge voltage waveform.
In accordance with a fourteenth aspect of the present invention, in the thirteenth aspect, voltage difference in the pre-discharge voltage waveform is set based on the length of the discharge pause period.
In accordance with a fifteenth aspect of the present invention, in the first aspect, the voltage of the pre-discharge voltage waveform is first decreased below the bias voltage of the electric-mechanical transducer so as to increase the volume of the ink chamber and thereafter increased above the bias voltage.
In accordance with a sixteenth aspect of the present invention, in the first aspect, the voltage of the pre-discharge voltage waveform is first increased above the bias voltage of the electric-mechanical transducer so as to decrease the volume of the ink chamber and thereafter decreased below the bias voltage.
In accordance with a seventeenth aspect of the present invention, in the first aspect, the voltage of the pre-discharge voltage waveform is decreased below the bias voltage of the electric-mechanical transducer so as to increase the volume of the ink chamber and returned to the bias voltage for N times just before the restart of the application of the discharge voltage waveform. The number N is determined by the pre-discharge voltage waveform determination means depending on the length of the discharge pause period.
In accordance with an eighteenth aspect of the present invention, in the first aspect, the voltage of the pre-discharge voltage waveform is increased above the bias voltage of the electric-mechanical transducer so as to decrease the volume of the ink chamber and returned to the bias voltage for N times just before the restart of the application of the discharge voltage waveform. The number N is determined by the pre-discharge voltage waveform determination means depending on the length of the discharge pause period.
In accordance with a nineteenth aspect of the present invention, there is provided a driving method of an on-demand ink jet printer head for driving a printer head of an on-demand ink jet printer according to image data and thereby letting the ink jet printer head discharge ink drops from its orifices so that printing according to the image data will be executed. The driving method comprises a discharge voltage waveform application step, a discharge pause period length determination step, a pre-discharge voltage waveform determination step and a pre-discharge voltage waveform application step. In the discharge voltage waveform application step, discharge voltage waveforms are generated corresponding to each orifice of the ink jet printer head based on the image data, and the discharge voltage waveforms are applied to electric-mechanical transducers corresponding to each orifice so that ink drop discharge will be executed according to the image data due to volume change of ink chambers corresponding to each orifice caused by movement of the electric-mechanical transducer and thereby an image according to the image data will be printed. In the discharge pause period length determination step, the length of each discharge pause period with respect to each orifice is determined based on the image data. In the pre-discharge voltage waveform determination step, a pre-discharge voltage waveform to be applied to the electric-mechanical transducer before restart of ink drop discharge after the discharge pause period is determined or selected based on the length of the discharge pause period which has been determined in the discharge pause period length determination step. In the pre-discharge voltage waveform application step, the pre-discharge voltage waveform which has been determined in the pre-discharge voltage waveform determination step is applied to the electric-mechanical transducer.
In accordance with a twentieth aspect of the present invention in the pre-discharge voltage waveform determination step of the nineteenth aspect, voltage difference in the pre-discharge voltage waveform is set larger than voltage difference in the discharge voltage waveform.
In accordance with a twenty-first aspect of the present invention, in the pre-discharge voltage waveform determination step of the nineteenth aspect, a discharge pause cycle number is calculated by dividing the length of the discharge pause period by the driving cycle of the ink jet printer head, and the pre-discharge voltage waveform is determined or selected based on the calculated discharge pause cycle number.
In accordance with a twenty-second aspect of the present invention, in the nineteenth aspect, the pre-discharge voltage waveform is applied to the electric-mechanical transducer just before the restart of the application of the discharge voltage waveform after the discharge pause period.
In accordance with a twenty-third aspect of the present invention, in the twenty-second aspect, the pre-discharge voltage waveform is applied to the electric-mechanical transducer a printer head driving cycle before the restart of the application of the discharge voltage waveform.
In accordance with a twenty-fourth aspect of the present invention, in the twenty-second aspect, the pre-discharge voltage waveform is applied to the electric-mechanical transducer two printer head driving cycles before the restart of the application of the discharge voltage waveform.
In accordance with a twenty-fifth aspect of the present invention, in the pre-discharge voltage waveform determination step of the nineteenth aspect, the determination or selection of the pre-discharge voltage waveform is executed so that ink meniscus vibration due to the application of the pre-discharge voltage waveform will be attenuated enough before the restart of the application of the discharge voltage waveform.
In accordance with a twenty-sixth aspect of the present invention, in the nineteenth aspect, the voltage of the pre-discharge voltage waveform is gradually increased from the bias voltage of the electric-mechanical transducer during the discharge pause period so as to decrease the volume of the ink chamber gradually and thereafter returned to the bias voltage just before the restart of the application of the discharge voltage waveform.
In accordance with a twenty-seventh aspect of the present invention, in the nineteenth aspect, the voltage of the pre-discharge voltage waveform is gradually decreased from the bias voltage of the electric-mechanical transducer during the discharge pause period so as to increase the volume of the ink chamber gradually and thereafter returned to the bias voltage just before the restart of the application of the discharge voltage waveform.
In accordance with a twenty-eighth aspect of the present invention, in the twenty-seventh aspect, the pre-discharge voltage waveform in the return to the bias voltage is set so that ink drop discharge will not be caused by decrease of the volume of the ink chamber.
In accordance with a twenty-ninth aspect of the present invention, in the nineteenth aspect, the voltage of the pre-discharge voltage waveform is decreased below the bias voltage of the electric-mechanical transducer so as to increase the volume of the ink chamber and thereafter returned to the bias voltage just before the restart of the application of the discharge voltage waveform.
In accordance with a thirtieth aspect of the present invention, in the twenty-ninth aspect, voltage difference in the pre-discharge voltage waveform is set based on the length of the discharge pause period.
In accordance with a thirty-first aspect of the present invention, in the nineteenth aspect, the voltage of the pre-discharge voltage waveform is increased above the bias voltage of the electric-mechanical transducer so as to decrease the volume of the ink chamber and thereafter returned to the bias voltage just before the restart of the application of the discharge voltage waveform.
In accordance with a thirty-second aspect of the present invention, in the thirty-first aspect, voltage difference in the pre-discharge voltage waveform is set based on the length of the discharge pause period.
In accordance with a thirty-third aspect of the present invention, in the nineteenth aspect, the voltage of the pre-discharge voltage waveform is first decreased below the bias voltage of the electric-mechanical transducer so as to increase the volume of the ink chamber and thereafter increased above the bias voltage.
In accordance with a thirty-fourth aspect of the present invention, in the nineteenth aspect, the voltage of the pre-discharge voltage waveform is first increased above the bias voltage of the electric-mechanical transducer so as to decrease the volume of the ink chamber and thereafter decreased below the bias voltage.
In accordance with a thirty-fifth aspect of the present invention, in the nineteenth aspect, the voltage of the pre-discharge voltage waveform is decreased below the bias voltage of the electric-mechanical transducer so as to increase the volume of the ink chamber and returned to the bias voltage for N times just before the restart of the application of the discharge voltage waveform. The number N is determined in the pre-discharge voltage waveform determination step depending on the length of the discharge pause period.
In accordance with a thirty-sixth aspect of the present invention, in the nineteenth aspect, the voltage of the pre-discharge voltage waveform is increased above the bias voltage of the electric-mechanical transducer so as to decrease the volume of the ink chamber and returned to the bias voltage for N times just before the restart of the application of the discharge voltage waveform. The number N is determined in the pre-discharge voltage waveform determination step depending on the length of the discharge pause period.
In accordance with thirty-seventh through fifty-fourth aspects of the present invention, there are provided machine-readable record mediums storing programs for instructing a microprocessor unit, etc. to execute processes for driving a printer head of an on-demand ink jet printer according to the driving methods of the nineteenth through thirty-sixth aspects of the present invention.